In the field of thin film fabrication, gridless ion sources have been known, for example from U.S. Pat. No. 4,862,032, as providing a broad area ion beam capable of providing ion assistance for the thin film deposition process.
Systems such as those described in U.S. Pat. No. 4,862,032 require relative large gas flows in order to initiate and maintain the ion current. Because of these large gas flows, the ion source is susceptible to transients that can occur in the vacuum chamber in which these systems operate. The control systems have therefore required circuitry to handle instabilities that may be generated. Any fluctuation within the vacuum chamber would cause a rapid drop in the anode voltage or gas flow or both, that prevented transient spikes from developing into a catastrophic event, such as arcing in the vacuum chamber. If the transient signal was large enough, the power supply would cut out altogether.
The process of generating the ion current is complex and depends on many factors. It can therefore be difficult to distinguish a catastrophic event from the usual fluctuations that occur in producing the ion current. For example, the onset of the ion current itself produces a transient signal. However, for best protection, prior art control systems have been over designed such that often the power supply to the ion source would cut out unnecessarily. The range of operation of the ion source was therefore limited to regions where the ion source was known to be fully stable.
The ion source control system therefore allows itself to be dictated to by the vacuum chamber environment in order to achieve stable operation, which can be frustrating to the user seeking to operate the ion source under a particular set of parameters. Furthermore, it can take of the order of several seconds for the ion source to achieve stable operation, both at the initial onset of the ion current, and when any perturbation of the chamber environment occurs. It is therefore in the interests of the user to operate the ion source conservatively in an attempt to prevent the power supply cutting out and to maintain stable operation. This provides a substantial limitation on the range of operation of the ion source.
Recent advances by the present applicant, as detailed in PCT/AU99/00591 and PCT/AU01/01548 the contents of which are herein incorporated by reference, have provided a gridless ion source having far greater gas ionization efficiency than was previously known. Greater gas efficiency allows the same ion current to be produced from a reduced gas flow with greater stability and more reliability in initiating the ion current. However, these systems have tended to operate with a power supply as is typical in the prior art, and thus have been known to cut out due to spurious events in the vacuum chamber even though those spurious events are not true catastrophic events.